The focus of the laboratory work this year has been on multidrug resistance with special emphasis on breast cancer, and the regulation of both expression and function of the P-glycoprotein gene. We have evaluated differentiating agents in breast cancer, colon cancer, and neuroblastoma. In each system differentiating agents have differing effects, ranging from downregulation of the gene to increased expression with decreased function. Simultaneously, we have studied levels of expression in patient samples, from patients on drug resistance reversal trials for refractory lymphoma and breast cancer.